1. Field of the Invention
The present invention relates to nitride semiconductor laser devices having a lasing threshold value corresponding to a low current value, and having an extended longevity, and semiconductor laser apparatus therewith.
2. Description of the Background Art
Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L184-186 reports a nitride semiconductor laser having a GaN substrate with crystal growth thereon. The GaN substrate was prepared using the facet-initiated epitaxial lateral overgrowth (FIELO) technique and the nitride semiconductor laser device had a ridge-geometric stripe with a width of 3 xcexcm. The document does not refer to the thickness of a residual film of the ridge-geometric stripe.
While the GaN substrate had a crystal defect density smaller for example than a conventional sapphire substrate, the device had a lasing threshold value corresponding to a high current value of 144 mA (with a threshold current density of 10.9 kA/cm2). The present inventors conducted an experiment, which has revealed that the high current value corresponding to the lasing threshold is attributed to the fact that the ridge-geometric stripe had an inappropriate width W and a residual film of the ridge-geometric stripe had an inappropriate thickness d. Furthermore, there is a corelation between the values of width W and thickness d and if at least one of width W and thickness d has an inappropriate value the current value corresponding to the lasing threshold value would not be reduced.
Herein, width W of the ridge-geometric stripe refers to a maximal width of the stripe as seen from a resonator""s end surface of the semiconductor laser, as shown in FIG. 4. As such, if a ridge-geometric stripe 130 as seen in cross section is a trapezoid, as shown in FIG. 1, width W of the ridge-geometric stripe corresponds to the width of the bottom side of the trapezoid (see FIG. 1). Furthermore, thickness d of the residual film of the ridge-geometric stripe will refer to the thickness as measured from an interface of a light emitting layer 46 and a p-type layer 45 to the bottom of the ridge-geometric stripe, as shown in FIG. 4. In FIG. 4, light emitting layer 46 underlies p-type layer 45 and overlies an n-type layer 47.
Furthermore in the present specification a light emitting layer generally refers to a layer formed of a plurality of well layers and a plurality of barrier layers. It should not noted, however, that if a single-quantum well structure is applied a light emitting layer is formed of a single well layer.
The present invention contemplates a nitride semiconductor laser device grown on a nitride semiconductor substrate, wherein the aforementioned width W of the ridge-geometric stripe and thickness of the residual film of the ridge-geometric stripe have their respective, appropriate values to allow a lasing threshold value corresponding to a current value reduced to provide laser operation over a long period of time.
To achieve the above objects, the present invention provides a nitride semiconductor laser device including: a nitride semiconductor substrate; and a layered portion corresponding to a nitride semiconductor film grown on the nitride semiconductor substrate, the layered portion including an n-type layer and a p-type layer and a light emitting layer posed between the n- and p-type layers, of the n- and p-type layers a layer opposite to the nitride semiconductor substrate with the light emitting layer posed therebetween serving as an upper layer having a stripe of 1.9 xcexcm to 3.0 xcexcm in width, the light emitting layer and the upper layer having an interface distant from a bottom of the stripe by 0 xcexcm to 0.2 xcexcm. Thus the width of the stripe and the thickness of the residual film of the stripe can each have an appropriate value to provide a threshold value corresponding to a reduced current value.
In the present invention preferably the layered portion and the nitride semiconductor substrate sandwich a low-temperature buffer layer grown at a low temperature and thus formed. Thus if the nitride semiconductor substrate does not have a satisfactory crystallinity or a satisfactory surface morphology the layered portion can be provided satisfactorily.
In the present invention preferably the light emitting layer has P or As added thereto. Thus, electron and hole mobility can be improved to provide rapid modulation. Consequently the nitride semiconductor laser device can have an improved self-pulsation characteristic.
In the present invention, preferably, of the n- and p-type layers a layer closer to the nitride semiconductor substrate serves as a lower layer including a crack prevention layer containing In. Thus a crack can be prevented.
In the present invention preferably the light emitting layer starts with one of well and barrier layers, followed by the well and barrier layers alternately stacked thereon, and ends with the one of the well and barrier layers to have a multi-layered structure and the light emitting layer is formed of a total of two to six layers. This can provide a threshold value corresponding to a reduced current value.
In the present invention preferably the light emitting layer contains an impurity selected from the group consisting of Si, O, C, Ge, Zn and Mg. Thus, light emission can be intensified.
In the present invention preferably the upper layer includes a carrier block layer containing Al. Thus, carriers can be firmly confined to provide a threshold value corresponding to a low current value.
To achieve the above object, the present invention provides an optical pickup apparatus including the nitride semiconductor laser device as described above. Thus the optical pickup apparatus can provide laser operation having a threshold value corresponding to a low current value and thus have an extended longevity.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.